Designing Circuit Boards in a High School Classroom
By Samuel Christy
Editor’s Note: Guest author Samuel Christy is an educator at Medford Vocational Technical High School. He focuses on robotics, electrical, and computer engineering.
Our high school engineering shop has a lot of advanced manufacturing tools, including a laser cutter, 3D printers, CNC mills and lathes, and even a water jet, but one of the most commonly used tools (on a weekly and sometimes daily basis) is the Bantam Tools Desktop PCB Milling Machine.
Our students work on a lot of independent projects. For each project, they’re expected to not just produce a proof of concept, but also a robust working prototype that will outlast their tenure in our program — something we can permanently display in the shop. One of the main features of a robust electronics prototype is that it includes a permanent circuit soldered on a custom circuit board.
By the middle of sophomore year, every student is able to design and manufacture their own microcontroller-based circuit board. For instance, this year’s sophomore class learned about circuit boards by designing and building acrylic LED displays.
Before constructing a circuit board, students are required to construct a complete circuit on a breadboard. Once this circuit is complete and tested, they can begin designing their circuit in a computer-aided design (CAD) program. We use EAGLE by Autodesk, which is free for schools.
The Bantam Tools Desktop PCB Milling Machine is so easy to use that there’s a tendency for students to try to cut a board without being fully confident in their design. In order to reduce the number of wasted boards, we require that students complete a fully operational prototype on a breadboard before starting CAD. In addition, any design changes that occur during the CAD phase must be reflected in their breadboard prototype. For example, if a student decides to add something as simple as a button or power switch, we require that they stop, make, and test this change on their breadboard before moving forward with their CAD. Finally, every design must be fully reviewed by a teacher before cutting the board.
Through a lot of experimentation, we’ve found what we consider the ideal settings for designing circuit boards in a classroom using the mill. In general, we have students follow the guidelines suggested on the Bantam Tools website. In addition, we require that students design only for a 1/32” bit unless their design demands a smaller bit. The 1/32” bits are powerful and make for easily solderable boards. Students use the 1/32” DRC file for EAGLE, provided by Bantam, but change the width setting to 18 mils to ensure all traces are 18 mils in width.
We’ve found that 18 mil-wide traces strike a good balance between being robust (they don’t tear off as easily) and relatively easily to route. We also require that all pads have the following dimensions:
Drill = 40
Diameter = 65
Shape = long
It took a while and a lot of frustrated students trying to solder pads that were way too small to find this sweet spot. One difficulty with this requirement is that just about every existing footprint needs to be modified.
Finally, we strongly suggest that students design for a single-sided board. As a result, students get really good at placing parts efficiently. In the cases where one or two wires simply cannot route, we have them pull the wires out from both end points to make jumping them with a wire easier. This can be seen in the image below. The vias are set to drill = 40, diameter = 65, and shape = round, so that they can easily be jumped using standard 22-gauge solid-core wire.
Finally, we recommend that students use right-angle mounts for all headers. This reduces the potential motion of the headers and reduces the likelihood that the copper pads will get separated from the circuit board. Simple holes in the board and zip ties can provide strain relief for cables.
Below is a finished shop project that uses the circuit shown above. The project is an X-Y plotter that can receive image files over wi-fi and print them in dots using a pen.
If you want to learn more about our program or have any questions, please check out our blog.